Regulation of γδ T cell survival by soluble HLA-I:: Involvement of CD8 and activating killer Ig-like receptors

We show that human V delta 1 or V delta 2 T lymphocytes secrete FasL and undergo apoptosis upon incubation with soluble HLA (sHLA) -I or after cross-linking of CD8, with a kinetics different from that observed following ligation of TCR. sHLA-1-induced apoptosis was blocked by anti-CD8 mAb; on the other hand, sHLA-1 was not effective in CD8- clones, while an HLA-I mutated in the alpha 3 domain, responsible for CD8 binding, was not functional on CD8(+) clones. Purified sHLA-Cw3 or -Cw4 alleles, isolated from the Cw3- or Cw4-transfected 721.221 lymphoblastoid cell line, triggered gamma delta T cell apoptosis, interacting with the specific receptors CD158j/KIR2DS2 or CD158 h/KIR2DS1, respectively, also known as activating isoforms of killer Ig-like receptors (KIR). Again, this effect was dependent on FasL secretion and it was blocked by specific mAb to KIR2DS2 or KIR2DS1. The engagement of CD8 or activating KIR also triggered the production of TNF-alpha. Noteworthy, sHLA-I molecules synergize with antigen-mediated activation of V delta 2 T cells: Indeed, V delta 2 T lymphocytes produced TNF-a when stimulated with isopentenyl pyrophosphate, and this effect was enhanced by sHLA-I. These findings suggest that sHLA-I can regulate gamma delta T cell survival and that activating KIR may amplify antigen-specific V delta 2 T cell responses.